From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750979AbVHQIQw (ORCPT ); Wed, 17 Aug 2005 04:16:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750980AbVHQIQw (ORCPT ); Wed, 17 Aug 2005 04:16:52 -0400 Received: from souterrain.chygwyn.com ([194.39.143.233]:6636 "EHLO souterrain.chygwyn.com") by vger.kernel.org with ESMTP id S1750978AbVHQIQv (ORCPT ); Wed, 17 Aug 2005 04:16:51 -0400 Date: Wed, 17 Aug 2005 09:25:52 +0100 From: Steven Whitehouse To: "Paul E. McKenney" , Suzanne Wood Cc: linux-kernel@vger.kernel.org, steve@chygwyn.com, walpole@cs.pdx.edu, patrick@tykepenguin.com Subject: Re: rcu read-side protection Message-ID: <20050817082552.GA25537@souterrain.chygwyn.com> References: <000f01c5a2bf$f8e752d0$6401a8c0@woodworkxi42l4> <20050817020156.GF1319@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20050817020156.GF1319@us.ibm.com> User-Agent: Mutt/1.4.1i Organization: ChyGwyn Limited Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Tue, Aug 16, 2005 at 07:01:57PM -0700, Paul E. McKenney wrote: > On Tue, Aug 16, 2005 at 05:09:29PM -0700, Suzanne Wood wrote: > [ . . . ] > > A read-side critical section is marked to protect the dereference of the > > dn_ptr and assignment to dn_db which is a pointer to a dn_dev. (struct > > net_device is defined in /linux/netdevice.h and its dn_ptr in > > /include/net/dn_dev.h) Should this rcu-protection be extended to the line > > following rcu_read_lock()? Even though use_long is a simple char, it > > appears to be a member of an rcu-protected structure. > > Looks to me that this could indeed be a problem -- the structure > pointed to by dn_db could potentially be freed immediately after the > rcu_read_unlock(), unless there is some other non-obvious locking > mechanism protecting it. In which case, why the rcu_read_lock() > and rcu_read_unlock()... > > Thanx, Paul The dev->dn_ptr points to the DECnet specific portion of a net device which is allocated in dn_dev.c/dn_dev_up and freed in dn_dev.c/dn_dev_delete when the net device goes up and down. So I think you are right in that as far as I can see, its possible for a net device going down to race with this, but the window of opportunity is very small indeed (in fact possibly zero?) due to the ordering of operations in dn_dev_delete where dev->dn_ptr is set to NULL (esentially preventing any more DECnet packets being received on that device) before flushing all neighbours and only then releasing dn_db. Also, Patrick Caulfield is maintaining this code now, so I've added him to the CC list. Thanks for the report though, Steve.